1. Field of the Invention
This invention has specific application in the field of man-made building products which simulate natural stone, although it is generally useful for any blending operation involving a motionless mixer wherein partial pigmentation involving two or more differently-colored streams of material is necessary or desired.
2. Description of the Prior Art
The beautiful appearance and extraordinary durability of marble have been recognized for centuries and have created great demand for its use in a multitude of contexts, many of them architectural. In recent years, the often prohibitive cost of marble has resulted in numerous attempts to simulate this natural stone using less expensive man-made materials. Products resembling marble have been produced by incorporating streams of pigment into filled resin bases. The material is generally made by combining a polymeric resin with an inert filler and thereafter adding and swirling in a pigment before gelation, or curing, of the material takes place.
The art indicated that at one time most producers of "marbleized" products swirled the coloring or veining pigments into the products manually in much the same random, uncontrolled way as two-flavor "marble" cakes are prepared. Such products lacked the relative uniformity of vein structure which characterizes real marble. An additional problem with manual swirling was the amount of time required to remove the already-curing polymeric resin from the automatic mixer in which the reactive additives had been added to a site suitable for manual mixing. Since marbleizing must be completed before the resin begins to crosslink, or cure, time is of the essence in these operations.
More recently, manufacturing methods dispensing with manual marbleization steps have been developed. For example, Duggins, U.S. Pat. No. 3,488,246, Jan. 6, 1970, discloses a simulated marble building product produced by a "relatively high speed continuous operation apparatus and process". In the Duggins process, streams of pigment having a relatively low viscosity, i.e., in the range of about 0.9 to about 3.0 poises, are added to a mass of polymerizing material having a relatively high viscosity, whereupon the viscous mass is subjected to low intensity mixing action to progressively displace the separate streams of pigment throughout.
Motionless mixers of the type disclosed by Grout et al., U.S. Pat. No. 3,800,985, Apr. 2, 1974, are useful in the commercial preparation of simulated marble products for a number of reasons. These mixers can be attached directly to the dynamic mixheads used to thoroughly blend reactive additives into the resin base, and can produce a streaked or striated (rather than homogeneously colored) product in a sufficiently short period of time. In the motionless mixer, material is rotated successively in alternating right- and left-handed directions transverse to the cylindrical conduit through which it travels. Between each such reversal, the material is subdivided into two streams. The flow at the center of each rotating segment is accelerated and the flow at the edges is decelerated, so that each particle of material is caused to travel continuously from the center of flow to the edges and back again. Only when the material is of homogeneous viscosity, however, is this the case.
In the methods of the prior art, it has been attempted to introduce into a motionless mixer two discrete streams: a low volume, low viscosity pigment stream and a high volume, high viscosity polymer stream. In such attempts to obtain marbleization, partial blending becomes a difficult task and the material must pass through many successive direction rotations in order to achieve a marbled appearance. Further, this marbled appearance has a tendency to vary significantly from batch to batch. These variations in appearance are troublesome in instances in which it is desired to match the products of one batch with that of another, for instance, where a simulated marble countertop is to be assembled from two or more separate pieces of material.
The prior art, as exemplified above and as it relates to the preparation of simulated marble building products, has an unfulfilled need for a mixing device that lends itself to current commercial practice and makes possible the attainment of marbled products having acceptably uniform striations and a consistently realistic appearance.